Meckel's cartilage
Encyclopedia
The cartilaginous bar of the mandibular arch is formed by what are known as Meckel’s cartilages (right and left) also known as Meckelian cartilages; above this the incus
and malleus
are developed.
The dorsal end of each cartilage is connected with the ear-capsule and is ossified to form the malleus
; the ventral ends meet each other in the region of the symphysis menti
, and are usually regarded as undergoing ossification
to form that portion of the mandible
which contains the incisor teeth.
The intervening part of the cartilage disappears; the portion immediately adjacent to the malleus is replaced by fibrous membrane, which constitutes the sphenomandibular ligament
, while from the connective tissue covering the remainder of the cartilage the greater part of the mandible is ossified.
Johann Friedrich Meckel, the Younger
discovered this cartilage in 1820.
from which the mandibles (lower jaws) of vertebrates evolved. Originally it was the lower of two cartilages which supported the first gill
arch (nearest the front) in early fish
. Then it grew longer and stronger, and acquired muscles capable of closing the developing jaw.
In early fish and in chondrichthyans (cartilaginous fish such as shark
s, which are not primitive
in any sense of the word), the Meckelian Cartilage continued to be the main component of the lower jaw. But in the adult forms of osteichthyans
(bony fish) and their descendants (amphibians, reptiles, birds, mammals), the cartilage was covered in bone - although in their embryos the jaw initially develops as the Meckelian Cartilage. In all tetrapods the cartilage partially ossifies (changes to bone) at the rear end of the jaw and becomes the articular
bone, which forms part of the jaw joint in all tetrapods except mammals
.
Incus
The incus or anvil is the anvil-shaped small bone or ossicle in themiddle ear. It connects the malleus to the stapes. It was first described by Alessandro Achillini of Bologna.The incus transmits sound vibrations from the malleus to the stapes....
and malleus
Malleus
The malleus or hammer is a hammer-shaped small bone or ossicle of the middle ear which connects with the incus and is attached to the inner surface of the eardrum...
are developed.
The dorsal end of each cartilage is connected with the ear-capsule and is ossified to form the malleus
Malleus
The malleus or hammer is a hammer-shaped small bone or ossicle of the middle ear which connects with the incus and is attached to the inner surface of the eardrum...
; the ventral ends meet each other in the region of the symphysis menti
Symphysis menti
The external surface of the mandible is marked in the median line by a faint ridge, indicating the symphysis menti, mandibular symphysis, or line of junction of the two pieces of which the bone is composed at an early period of life....
, and are usually regarded as undergoing ossification
Ossification
Ossification is the process of laying down new bone material by cells called osteoblasts. It is synonymous with bone tissue formation...
to form that portion of the mandible
Mandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...
which contains the incisor teeth.
The intervening part of the cartilage disappears; the portion immediately adjacent to the malleus is replaced by fibrous membrane, which constitutes the sphenomandibular ligament
Sphenomandibular ligament
The sphenomandibular ligament is a flat, thin band which is attached above to the spina angularis of the sphenoid bone, and, becoming broader as it descends, is fixed to the lingula of the mandibular foramen...
, while from the connective tissue covering the remainder of the cartilage the greater part of the mandible is ossified.
Johann Friedrich Meckel, the Younger
Johann Friedrich Meckel
Johann Friedrich Meckel , often referred to as Johann Friedrich Meckel, the Younger, was a German anatomist born in Halle...
discovered this cartilage in 1820.
Evolution
The Meckelian Cartilage, also known as "Meckel's Cartilage", is a piece of cartilageCartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
from which the mandibles (lower jaws) of vertebrates evolved. Originally it was the lower of two cartilages which supported the first gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
arch (nearest the front) in early fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
. Then it grew longer and stronger, and acquired muscles capable of closing the developing jaw.
In early fish and in chondrichthyans (cartilaginous fish such as shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....
s, which are not primitive
Primitive
Primitive may refer to:* Anarcho-primitivism, an anarchist critique of the origins and progress of civilization* Primitive culture, one that lacks major signs of economic development or modernity...
in any sense of the word), the Meckelian Cartilage continued to be the main component of the lower jaw. But in the adult forms of osteichthyans
Osteichthyes
Osteichthyes , also called bony fish, are a taxonomic group of fish that have bony, as opposed to cartilaginous, skeletons. The vast majority of fish are osteichthyes, which is an extremely diverse and abundant group consisting of over 29,000 species...
(bony fish) and their descendants (amphibians, reptiles, birds, mammals), the cartilage was covered in bone - although in their embryos the jaw initially develops as the Meckelian Cartilage. In all tetrapods the cartilage partially ossifies (changes to bone) at the rear end of the jaw and becomes the articular
Articular
The articular bone is part of the lower jaw of most tetrapods, including amphibians, sauropsids and early synapsids. In these animals it is connected to two other lower jaw bones, the suprangular and the angular...
bone, which forms part of the jaw joint in all tetrapods except mammals
Evolution of mammals
__FORCETOC__The evolution of mammals within the synapsid lineage was a gradual process that took approximately 70 million years, beginning in the mid-Permian. By the mid-Triassic, there were many species that looked like mammals, and the first true mammals appeared in the early Jurassic...
.